Isotropic diffusion weighting using a triple-stimulated echo pulse sequence with bipolar gradient pulse pairs

• Microscopic diffusion anisotropy for pore liquids with short T2.
• Immune to background magnetic field gradients.
• Component selective through chemical shift resolution.
• Applicable to inorganic porous solids, soft matter, and biological tissues.

Microscopic diffusion anisotropy in porous materials can be quantified from diffusion NMR data acquired with a combination of directional and isotropic diffusion encoding. A drawback with current pulses sequences for isotropic encoding is that they all rely on spin echo sequences, which are only applicable to pore liquids with long transverse relaxation times and porous materials with negligible internal magnetic field gradients. To mitigate these problems, we introduce a pulse sequence based on consecutive stimulated echo blocks with bipolar gradient pulse pairs giving equal diffusion encoding in three successive directions. By varying the angles between these directions, the pulse sequence can be tuned to give either directional or isotropic diffusion encoding. We demonstrate the new pulse sequence by experiments on detergent/water liquid crystals with lamellar, bicontinuous cubic, and reverse 2D hexagonal structures.

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